First, decide what do you want to experience or see in Manali. I am going to suggest the best time according to my personal experience. I have divided it into 3 categories according to the Manali weather. Weather plays an important role to give you stupendous experience.
Mid-March to end of June (moderate temperature)
From March to June its charm is exceptional. It is the best time to visit Manali if anyone wants to enjoy adventure activities like river rafting, paragliding, trekking, and camping. You will get a pleasant climate with temperatures ranging between 10 degree Celsius and 25 degree Celsius.
There is an ample number of tourists approaching this hill station. The summer vacations of schools and sweltering heat in plain areas cause families to come and enjoy the Manali weather.
You can go to Rohtang Pass, which is the most awaited place to get open during this time, preferably after Mid-May. Go for Paragliding and other sports in Solang Valley. You might get a chance to go for skiing in Solang during 1st or 2nd Week of March, that too depends on Weather.
Go for treks to higher passes like Chandarkhani Pass, Bhrigu Lake, Hampta Pass, etc.
One thing will annoy you that is a Traffic jam on Rohtang Pass as this currently it is the only way to go Leh Ladakh or Lahaul & Spiti. During this time, you also face a hike in hotel & cab prices and parking issue, if you are traveling by your vehicle. If you love crowded tourists places with trekking/camping then this time is good for you.
July to October (monsoon and autumn)
Clouds start pouring in Manali and Beas River puts forth its infuriated current. Manali is dangle under clouds floating over the sky. It redefines the luster of the place. Rains here can last up to days, providing a much-required boost to the water streams and rivers. Activities are also limited during this time and considered this as the offseason in Manali but still, there is a lot on a showcase of nature to see and wonder.
Monsoon months in Manali are from July to September, experience relentless rainfall, heavy rainfall occasionally triggers landslides which makes traveling in the dangerous. It is suggested to stay away from Manali at this time. You might not enjoy Manali in monsoon.
Despite rainy season, you can go to Rohtang Pass, Vashisht temple for bathing in hot springs and other treks nearby Manali. You might get no adventure activity in Solang or Kothi. If you see according to cost-effectiveness, then this time is good to visit Manali.
November to March (Frigid Winters)
Temperature often drops to -5°C accompanied by snowfall in December & January. Starting from December to February, everything Mountains, Roads, Temples covers with a white sheet of snow. Manali becomes a paradise on earth at this time.
During this time you can enjoy fresh snow and snowy activities. This is the best time if you want to see fresh snow or enjoy snow-related activities like skiing and skating. You can do some snow treks like Bhrigu lake, Beas Kund, etc.
You can roam around old Manali and enjoy open bathing in hot springs of Vashisht temple at sub-zero temperature. Manali is preferred as an ideal place for honeymooners during this time because of awesome manali weather.
You can’t get everything positive, so at this time, you might have to spend the whole day inside your hotel room due to heavy snowfall. And also don’t try to drive on the roads which are prohibited at the time of snow. No one can go to Rohtang Pass at this time of the year as roads are closed.
Nature is magnificent in every phase, you just need to decide which phase you admire most.